1. Field of the Invention
The present invention relates to centrifuge tubes and, more particularly, to the sealing or closing of centrifuge tubes.
2. Description of Related Art
Typical centrifuge tubes have a generally uniform cylindrical shape with one end having an opening to receive the fluid sample to be subjected to centrifugation. After the introduction of the fluid sample into the tube, it is usually necessary to provide a very tight closure or capping means over the open-end of the tube during centrifugation. The closure or capping mechanism must be extremely tight, especially when the tube is used in a centrifuge rotor of the type known as a fixed-angle rotor in which the tube cavities are oriented at an acute angle or approach an angle of 0.degree. with respect to the spin axis of the rotor. Hydrostatic pressure within the tube becomes extremely great when the centrifuge is rotated at speeds of 20,000 rpm or greater.
A continual problem with the placement of capping means on the open-ended centrifuge tubes is insuring that a proper seal is being achieved between the plug and the tube to prevent any possible or potential leakage which could occur. Although swinging bucket type rotors do not require capping means to establish a seal over the top of the centrifuge tube because the centrifugal forces are directed toward the bottom of the bucket, sealing concerns increase as the angle of orientation of the tube axis with respect to the spin axis is less than 90.degree.. The most significant concern for sealing the centrifuge tube occurs when the angle of the tube with respect to the spin axis is zero or essentially vertical.
The significance of eliminating or preventing any potential leakage in a high speed centrifuge cannot be underestimated, because the fluid sample may contain some type of pathogen, mutagen, bacteria or some valuable type of material which the user does not wish to lose through leakage during the centrifugation run. Further, any leakage which may occur during the centrifugation run will invalidate the run resulting in great inefficiency in the use of the centrifuge. Since many of the fluid samples which are investigated during the centrifuge run contain important ingredients for use by a scientist or technician, the leakage of a centrifuge tube during a run can create significant problems as well as the waste of an operator's time. Moreover leakage will cause rotor imbalance and result in rotor mishap.
In many instances, the resulting leak is caused by an improper seal being achieved between the capping means and the centrifuge tube because of either a poor configuration or design of the capping means or as a result of the improper placement of the capping means on the tube. Not only is it important that the capping means be designed to achieve a secure seal between the test tube and the capping means, but also it is important that the capping means have such a design that it is easy to remove after the centrifuge run without having to disturb the contents of the fluid sample after the centrifugation. Otherwise, the sample constituents may be remixed and invalidate the centrifugation run.
Various configurations have been devised for capping centrifuge tubes to eliminate any potential leakage which may occur. Typically, the capping means is utilized on a disposable type of centrifuge tube which is somewhat flexible. Present capping means are typically very complicated in their manufacture and construction as well as in their use and application for attachment to the tube. Many of the capping means used are designed to tightly grip the open-end of the centrifuge tube which is flexible or pliable and conform the open-end of the tube to the gripping portion of the capping means. However, the open-end of the tube must be inserted properly and completely within the capping means in order to achieve a secure seal when the capping means is tightened. Unfortunately, the tube often is not completely inserted within the capping means so that when the plug is tightened, a proper seal is not achieved.
U.S. Pat. No. 4,537,320 describes a capping means which includes a support crown for fitting around the filler stem of the centrifuge tube and a threaded swage plug which threads within the support crown to cause a swage seal to be formed between the filler stem and the plug. U.S. Pat. No. 4,690,670 describes the use of a self-tapping screw plug for sealing a plastic centrifuge tube. Both of these sealing means require hand tools for tightening the threaded plugs. It is often difficult to apply the right amount of torque when the plugs are tightened. Insufficient torquing results in an inadequate seal against high hydrostatic pressure in the tube while over-torquing results in stripping of the threads of the plastic components which will destroy the sealing function of the plugs.
U.S. Pat. No. 4,301,963 discloses the sealing of the narrow stem of an integral one-piece centrifuge tube by fusing the plastic material of the neck. U.S. Pat. No. 4,285,904 discloses sealing a narrow stem centrifuge tube by fusing a plug member over the neck of the centrifuge tube. While these methods produces good seals for narrow stem centrifuge tubes, the methods are not suitable for wide stem centrifuge tubes. It is sometimes preferable to use wide stem tubes over narrow stem tubes because it is easier to load and unload the tubes with sample solution. The wider opening allows the use of larger diameter pipettes and syringes. For biological samples made up of large molecules, the larger diameter instruments causes less shear on the large molecules being transferred into or out of the wide stem centrifuge tubes, thereby less damage to the sample. Furthermore, the centrifuge tubes that are heat-sealed can only be used once since the sealed stem is cut off before the sample is removed after centrifugation. Still further, the heat seal technique requires special sealing tools including heaters. Thus, the cost for practicing heat sealing of centrifuge tubes is expensive compared to mechanical capping methods.